This application pertains to low-shrink molding resin systems based on thermosetting polyester polymers and relates to commonly assigned U.S. Pat. No. 3,883,612 issued on May 13, 1975, which is incorporated herein by reference.
Unsaturated thermosetting polyester polymers blended with vinyl monomers such as styrene are well-known molding resins that can be cured at room temperature or under heat and/or pressure to form a thermoset plastic molded part. Unsaturated polyester molding resins often are combined with inert fillers, glass fibers, glass flakes, clays and talcs, and the like for the purpose of obtaining improved impact strength, flexural strength, and rigidity in the molded part. Most conventional thermosetting polyester resins, however, characteristically shrink about 8 to 10% by volume and consequently distort during the molding process whereby the shrinkage causes unsatisfactory molded parts parts particularly for automotive parts. To offset the shrinkage characteristic of thermosetting polyester resin systems, low-shrink resin systems have been suggested based primarily on the inclusion within the thermosetting polyester resin system of a thermoplastic polymer such as an acrylic polymer wherein the thermoplastic acrylic is ordinarily incompatible with the liquid matrix thermosetting polyester resin and subsequently cures to a heterogeneous solid. A particularly desirable low-shrink thermosetting polyester resin composition is a stabilized one component system disclosed in U.S. Pat. No. 3,883,612 which provides a dicyclopentadiene modified thermosetting polyester resin containing a vinyl-acrylic thermoplastic uniformly dispersed therein whereby the low-shrink resin cures to lowprofile solid having the thermoplastic polymer uniformly distributed within the thermoset matrix phase.
It now has been found that an improved low-shrink thermosetting polyester resin system can be achieved by dispersing a linear, polyurethane within a dicyclopentadiene modified polyester resin to produce a stabilized, one component, uniform dispersion mixture that can be cured to form a low-profile molded part.